The most serious and awful surprise: a terror attack in the heart of Paris.

This is what many political observers have been dreading.

Less than two weeks ago a well-connected political observer in Paris told me a terror attack just before the election "would give Le Pen the presidency".

Marine Le Pen, the far-right National Front candidate, has been explicit in her message: that the political establishment has not done enough to counter Islamist extremism.

She proposes more police officers, more prisons. She wants to slam the doors on immigration, against refugees. She wants to deport any criminal who is not a French citizen.

At a rally I attended a week ago, she promised a rapt audience she would be a president who would "preside and protect" France against the "immense menace" of Islamist terrorism.

Her opponents "will not protect our children from terrorists", she said. The security of the public "is my main mission".

"We must not get used to the dramas ... of Islamist terrorism," she said, citing the list of attacks in France as well as in Berlin, London and Brussels.

France has been in a state of emergency since the attacks of 2015. As well as the horrific attack in Nice last year it has seen many 'near misses' - there have been regular raids and arrests across the country.

Just days ago two men were arrested and guns and explosives (and an Islamic State flag) seized in Marseilles, foiling what was described as an "imminent" plot to attack one of the presidential candidates.

Le Pen is not the only candidate who had campaigned on security against terrorism.

Francois Fillon, the Republican candidate who should have had the election in the bag if it weren't for a nagging corruption scandal, has mainly focused on economic issues.

But he also rails against the "totalitarianism of Islamism" and promises a "serious fight against crime" which would include deporting foreign terrorists. He plans to reduce immigration to a "strict minimum", though he doesn't frame this as an anti-terror policy. At a rally in Lyon a week ago, I heard one of the biggest cheers of the night when he promised to "restore the security of France".

The common assumption is that a terror attack would boost both campaigns - especially Le Pen's. Le Pen, says one diplomatic source, has the capacity to project herself as a "Joan of Arc figure" who alone can save the country.

However - while this theory makes sense - it may not actually translate to votes in Sunday's first round of the presidential vote, let alone the second round when it is likely to be Le Pen versus a candidate closer to the political centre.

"Somebody will usually say 'well terrorism changes everything and if there is an attack everyone will vote for Le Pen'," says Matthew Goodwin, professor of politics at the University of Kent. "But if you go back to the Paris attacks and you look at the polling before and after the two big Paris attacks, Le Pen did not receive a boost after those attacks.

"In the same way, in other Western democracies when there's been a serious terrorist attack, populists in general have not benefited and I think that's an important point."

Indeed, looking back to the Nice attack in mid-July 2016, just before the attack Le Pen was polling 28 per cent. The next poll, in early September, shows her vote unchanged - if anything slightly lower.

And the attacks in Paris gave a short-lived boost to President Francois Hollande's popularity.

The French have not been jolted out of complacency by this attack - there is no such complacency. They see the regular raids and arrests on the news every week. They are well aware of the threat.

But it is hard to imagine how this attack could fail to boost Le Pen's claims of an "explosion of violence" in France that needs an uncompromising response.